This invention relates generally to apparatus and processes for aerating dispersions. More particularly, the present invention relates to apparatus and processes for aerating pulp suspensions during de-inking.
De-inking flotation is a mechanical process for removing impurities and ink particles from pulp suspensions produced particularly in waste paper treatment. Here hydrophobic solids, such as ink particles or stickies, are brought into contact with gas bubbles in a suitable flotation cell. The adhering solids are carried to the surface of the liquid together with the bubbles and can be removed there as scum. The pulp is discharged with the accept due to its hydrophile nature. To generate the gas bubbles, one of the devices used successfully for this purpose is the self-priming injector. Since the process for selective removal of printing ink from pulp suspensions only operates with low efficiency, multi-stage processes, as known, e.g. from DE 41 16 916 C2, are normally used. Here the entire pulp suspension is fed through injectors in order to create a bubble surface large enough for successful flotation. The accept from each stage is used as feed to the following stage for intake of the gas flow by injectors. The scum from all stages is collected and can then be floated once again in a secondary stage in order to recycle the fibers. The disadvantage of processes of this kind is that they are susceptible to production fluctuations, have no means of adapting the air content to the specific application and require a great deal of measuring and process equipment to run the secondary flotation stage.
The invention is, therefore, based on the task of designing a multi-stage energy-efficient process for flotation of pulp suspension which is insensitive to fluctuations in production and can be adapted easily to different applications and compositions of raw material.
The invention is thus characterized by each stage having a separate liquid circulating system which sucks in the gas or air and mixes it with the liquid. Since the feed and injector propulsive jet used to suck in the gas do not interact, an additional degree of freedom is formed which makes it possible to set the air content independently of the feed rate. As a result the plant can be adapted very easily to changes in feed volume flows.
An advantageous further development of the invention is characterized by the air content being controlled independently of the feed rate. With this control facility the air content can be adapted easily to changes in feed conditions.
A favorable configuration of the invention is characterized by the air feed rate being set according to the properties of the pulp suspension. In this way optimum flotation success can be obtained for each individual application.
A favorable further development of the invention is characterized by different quantities of air being applied to the individual stages. Thus, special raw materials can also be floated efficiently without influencing the raw material feed and thus, the throughput.
An advantageous configuration of the invention is characterized by the suspension being fed to the next cell/stage in the lower section of the cell/stage. Feeding the suspension in at the lower section of the cell/stage prevents the impurities consolidated in the scum from being discharged with the accept.
A favorable further development of the invention is characterized by at least one secondary stage being provided, which is operated with the overflow from the primary stage and whose accept is fed to the first primary stage. As a result, low fiber losses can be obtained in flotation of pulp suspensions, where the secondary cells can have the same design as the primary cells.
The invention also refers to a device for aerating dispersions, particularly a flotation device, for example for de-inking pulp suspensions, with at least two flotation cells, characterized by a separate injector being provided in each stage and which is connected to an assigned pump to form a liquid circulating system. This provides a favorable means of excluding any interaction between feed and injector propulsive jet, thus forming an additional degree of freedom.
A favorable further development of the invention is characterized by openings being provided in the cell dividing walls at floor level. This arrangement allows the dispersion to flow into the next cell in each case, but prevents the scum from being carried along with it.
An advantageous configuration of the invention is characterized by overflow channels being provided in each cell and which are connected to a collecting tank via a collecting main. In this way the scum from each of the primary cells can be collected and either discharged or subjected to further treatment all together.
A favorable configuration of the invention is characterized by at least one secondary flotation cell, operated with the overflow front the primary cells and whose accept is fed to the first primary cell, being provided and by this secondary flotation cell having a connection to the collecting tank. By using a secondary flotation cell upstream of the primary flotation, fiber losses can be reduced even further.
A favorable further development of the invention is characterized by at least two cells being arranged in a tower. Such an arrangement permits even better separation of impurities from the accept.
An advantageous further development of the invention is characterized by at least three cells being combined to form a circular flotation plant. With this arrangement, good separation of impurities can be achieved with a low space requirement.
A favorable further development of the invention is characterized by several injectors being provided in each cell. If several injectors are provided, particularly at different points within a cell, this allows higher throughput on the one hand, and on the other it permits better distribution of the suspension and of the liquid bubbles.